The present invention is directed to a device for positioned plate-shaped pieces such as a board or paper sheet within a processing machine and especially within a positioning device during the pieces travel on the machine feed table so that the sheet is aligned against the front and side stops by means of conveying devices such as rollers or elastic belts before the sheet or board is carried into the carrier such as grippers which are fitted on cross bars and are themselves permanently mounted on an endless chain of the processing machine. Generally, such a feed table is arranged upstream of the processing station which may be the cutting and creasing station of a platen press.
In the processing of unprinted plate-shaped pieces traveling through the platen press, it is sufficient, though necessary to have the use of front and side stops which are arranged exactly in line with the reference points of a subsequent processing station. In this way, the piece can be quickly moved forward against the stops by the conveying device and then seized by the carrier. The front stops are then retracted and the carrier thus is enabled to pull the piece into the processing station.
If the plate-shaped piece has previously undergone one or several printing operations and if the subsequent processing operations are to be carried out in a congruent position with regard to the previous printed positions, such a positioning method becomes inadequate.
In fact, notwithstanding precautionary measures, there are frequently errors or regular deviations of the position of a process part with regard to the front and lateral edges of the sheet. For example, after successive alignments against front and side stops in different areas of the edge, which is not always dependably straight or else after alignment occurs regularly in the same area, there are often caused impressions which will subsequently lead to misalignment during subsequent alignment steps. Thus, problems always occur when die cutting of pre-printed pieces to insure a positioning of the die cut relative to the printing to obtain a finished product of the desired quality.
As outlined in the description of French Patent No. 1,470,054, an answer to this problem consists in printing on the plate-shaped piece an additional mark consisting of a black crosswise line with a specific width and arranged in the middle of a white area of a sufficient length. When the plate-shaped piece is carried through, this mark travels through an area specifically illuminated by a light source where it is scanned by five photoelectric cells arranged on a crosswise line. An electrical logic system connected to these photocells will monitor the passage of such a mark by checking the various simultaneous conditions and cause the emission of a control signal in direct relationship with the lengthwise print position. However, such a device involves numerous drawbacks. For example, it only suggests the front position with an inadequate precision on account of the width of the scanning beam of the photocells and especially this device will require a compulsory imprint of an additional mark in the middle of the sheet at an adequate spacing between the areas being process which is not always feasible when optimum utilization of the plate-shaped piece surface requires a staggered or shingled arrangement of the sheets.